The invention relates to reinforced thermoset plastics having improved physical characteristics. More specifically, the invention relates to fiber reinforced polyester resin molding compounds having increased toughness and surface smoothness.
Reinforced thermoset plastics are made of plastic resins and reinforcing fibers. They are widely used in commerce as materials because of their advantageous combination of properties, including light weight, high strength to weight ratio, and ease and versatility of construction.
Polyester resins are common components of reinforced thermoset plastics. Such resins are based on a polyester polymer containing at least one unsaturated carboxylic component to introduce unsaturation into the backbone chain of the polymer. The resins further contain an unsaturated monomer. Such resins can undergo cure to form solid thermoset articles. Typically the heat required for cure is provided during compression molding of the article. Reinforcing fibers such as glass fibers and non-fibrous fillers can be added to the polyester resins to produce cured articles having a wide range of desirable physical properties.
Sheet molding compound is used in the manufacture of large reinforced compression moldings. Sheet molding compound is finding increased use in automotive applications that take advantage of its light weight, corrosion resistance, and design flexibility. In particular, sheet molding compounds find use as structural components for front end panels, tail gates, exterior and interior panels, hoods, roofs, deck lids, fenders, doors, and horizontal panels.
Sheet molding compound is a glass fiber reinforced thermoset polyester compound in sheet form. Because of its high strength, it is widely used in commerce as a construction material. Its strength, stiffness, and other properties make it suitable for use in horizontal as well as vertical panels, for example, in automobiles. Such panels on automobiles must display a high degree of surface smoothness so the painted surface is acceptable to the consumer.
Bulk molding compound is also based on polyester resin, but generally contains shorter reinforcing fibers and relatively more non-fibrous filler than sheet molding compound. Because of this, it has lower mechanical strength than sheet molding compound. Nevertheless, surfaces made of bulk molding compound are often exposed, and therefore, should be as uniform as possible for acceptance by the consumer.
Painted or topcoated products made from molded fiber reinforced thermoset polyester resins are subject to a number of defects. Such defects include microcracks in the article itself and paint pops on the surface of the article. Some of the defects are caused by improper use and installation of the article by the consumer, and others are inherent in the material itself.
A particular kind of defect is known as edge pop. Edge pops manifest themselves as craters or pops along the edge of the molded article. They are believed to result from the release of gaseous materials from the thermoset resin during cure. In practice, the problem is observed to be more severe at the edge of the article. It is believed this is due to the relative lack of reinforcing fibrous material at the edge of the article, or alternatively to the fact that the orientation of the fibers at the edge is not as uniform as elsewhere in the bulk of the article.
It would be desirable to increase the toughness of the resin in the thermoset article in order to reduce or eliminate the defect such as edge pops. At the same time, the strength of the article must be maintained.
Unsaturated poly resins made by polymerizing propylene glycol and maleic anhydride are widely used to produce reinforced compression moldings and sheet molding compounds. However, the articles suffer from the defects mentioned above.
Unsaturated polyester resins comprising copolymers of unsaturated carboxylic acids or anhydrides and a diol component including polypropylene oxide are known, but such resins have not been shown to be useful to produce rigid structural panels such as those produced from sheet molding compound or bulk molding compound. For example, in Japanese Patent 02150456 to Dainippon Ink and Chemicals, a sheet with good low temperature flexibility is produced that contains in addition to glass fiber reinforcement, a polyester polyether copolymer of ethylene glycol, propylene glycol, polypropylene oxide, phthalic acid, and maleic anhydride. The polypropylene oxide makes up 60% of the total amount of ethylene glycol, propylene glycol, and polypropylene oxide. It was not shown to be useful to make rigid panels.
In Japanese Patent 01062322 to Hitachi Chemical Co., a composition is used without glass fiber reinforcement to make a pipe lining. The composition is based on an unsaturated polyester that is a copolymer of alpha, beta unsaturated dibasic acids, unsaturated fatty acid oligomer, and polyalkylene glycol or alkoxylates of bisphenol A of number average molecular weight 700-2000. This polyester resin contains no monomeric diol, and produces a flexible material suitable as a lining.
The present invention provides a molding compound containing a reinforcing fiber, a non-fibrous filler, and an unsaturated polyester resin. The unsaturated polyester resin contains an unsaturated monomer as well as a polyester polymer. The polymer is a unsaturated polyester made from a hydroxyl component and a carboxylic component. The hydroxyl component contains both a monomeric diol or triol, and a polymeric polyol. The polymeric polyol is a hydroxyl-terminated polymer such as polyether or polyester diols or triols. The carboxylic component can be provided by an unsaturated carboxylic diacid, an unsaturated carboxylic anhydride, an unsaturated carboxylic diester or mixtures thereof.
In a preferred embodiment, the molding compound also contains a second fiber, with fibers shorter than those of the reinforcing fiber.
In another embodiment, molded articles are provided by compression or extrusion molding of a molding compound as described above. The articles are characterized by improved crack resistance, toughness, and surface appearance.